Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2008, Article ID 547301, 6 pagesdoi:10.1155/2008/547301

Research ArticleUV-Activated Luminescence/Colourimetric O2 Indicator

Andrew Mills,1 Cheryl Tommons,1 Raymond T. Bailey,1 M. Catriona Tedford,2 and Peter J. Crilly2

1 Department of Pure and Applied Chemistry, Westchem Graduate School of Chemistry, University of Strathclyde,295 Cathedral Street, Glasgow G1 1XL, Scotland

2 Chemical and Biological Sciences, Bell College of Technology, University of the West of Scotland, Hamilton ML3 0JB, Scotland

Correspondence should be addressed to Andrew Mills, a.mills@strath.ac.uk

Received 26 July 2007; Accepted 6 November 2007

Recommended by Russell Howe

An oxygen indicator is described, comprising nanoparticles of titania dispersed in hydroxyethyl cellulose (HEC) polymer filmcontaining a sacrificial electron donor, glycerol, and the redox indicator, indigo-tetrasulfonate (ITS). The indicator is blue-coloured in the absence of UV light, however upon exposure to UV light it not only loses its colour but also luminesces, unlessand until it is exposed to oxygen, whereupon its original colour is restored. The initial photobleaching spectral (absorbance andluminescence) response characteristics in air and in vacuum are described and discussed in terms of a simple reaction schemeinvolving UV activation of the titania photocatalyst particles, which are used to reduce the redox dye, ITS, to its leuco form, whilstsimultaneously oxidising the glycerol to glyceraldehye. The response characteristics of the activated, that is, UV photobleached,form of the indicator to oxygen are also reported and the possible uses of such an indicator to measure ambient O2 levels arediscussed.

Copyright © 2008 Andrew Mills et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. INTRODUCTION

The presence of oxygen in food packaging usually has adetrimental effect on the food products contained therein.Oxygen does not only react chemically with food tocause oxidative rancidity, but moulds, growths, and aerobicmicroorganisms—which discolour and decompose food andmake it offensive to smell and dangerous to eat—thrivein the presence of oxygen [1]. Therefore, it is no surprisethat the removal of oxygen in the food packaging industryis of immense importance. This is usually achieved viamodified atmosphere packaging (MAP), a process in whichthe atmosphere within the food package is flushed andreplaced with an inert gas, such as nitrogen or carbondioxide, often combined with an efficient oxygen scavenger,resulting in an oxygen level of 0.1% or less within the foodpackage [1, 2]. MAP increases the shelf life of many foodproducts by a factor of 3-4 compared to that in air, makingit a popular method of packaging food in the wholesale andretail food packaging industry.

There are many established methods for the detectionof oxygen, which include the Clark electrode [3] and gaschromatography [4], however, such methods are too expen-sive and time consuming to allow 100% quality assurance.

Consequently, there is an increasing interest in the develop-ment of cheap, easy-to-use oxygen indicators [5]. This area ofresearch has been dominated by the quenching of a polymer-encapsulated lumophore, such as ruthenium(II)-tris(4,7-diphenyl-1,10-phenanthroline), Ru(dpp)3

2+, by oxygen. Oneof the few commercially-available products based on thisapproach is the OxySense system [6], whereby, Ru(dpp)3

2+

is encapsulated in silicone rubber dots, called O2xyDotsTM,which can be attached to the inside of a package or bottle.The O2xyDot is illuminated and the readily measuredluminescence lifetime of the lumophore is equated to theoxygen level within the package. Unfortunately, the detectionof oxygen using the OxySense system, or any opticalsensor based on luminescence, requires the use of relativelyexpensive instrumentation for making the required lifetimeor intensity measurements.

Unlike changes in luminescence intensity or lifetime, asensor that changes colour in the presence of oxygen wouldbe most desirable for MAP, given that the human eye canthen act as the detector. Such indicators can take severalforms, such as a tablet [7, 8] or a printed layer [9, 10]. Thistechnology is typified by the Ageless Eye oxygen indicator,manufactured by the Mitsubishi Gas Company in Japan [7,8, 11], that comprises a redox-indicator, usually methylene

2 International Journal of Photoenergy

O2

H2O

TiO2 TiO2∗ (e−, h+)

hν > 3.2 eV

Glycerol

Glyceraldehyde

Step 1

Step 2

DOx

DRed

TiO2−

Figure 1: Schematic illustration of the key processes involved inthe UV activation and subsequent response towards oxygen of aTO2/redox dye, DOX/glycerol/HEC oxygen indicator, such as thatused in this work, where DOX = ITS.

blue, which, in the absence of oxygen, is maintained in itscolourless, chemically reduced, leuco form, by a reducingagent, such as glucose in an alkali medium. In the presence ofoxygen leuco, methylene blue is oxidised to a highly colouredform. All components are mixed together, along with anonredox dye, such as Acid Red 52, which provides a pinkbackground colour, and pressed together to form a pellet,which is subsequently encapsulated in an oxygen-permeable,ion-impermeable plastic sachet to avoid any contact with thefood. In the presence of oxygen, the Ageless Eye indicatorchanges from pink to purple in 2-3 hours, in a quasireversibleprocess. However, this indicator needs to be stored andhandled under anaerobic conditions. Such oxygen indicatorsare used in the food packaging industry, but mostly used as aresearch tool or fault diagnostic, since they have limited useelsewhere, because of cost and storage issues.

Lee et al. [12, 13] recently developed a new rangeof colourimetric oxygen indicators that are irreversible,reusable, and UV-light activated. Such “intelligent ink”oxygen sensors comprise a UV-absorbing semiconductor,such as TiO2, a redox-indicator, such as methylene blue, asacrificial electron donor, such as triethanolamine, and anencapsulating polymer such as hydroxyethyl cellulose; theingredients are mixed together, with water as the solvent, toform an ink. The ink can be coated or printed subsequentlyonto a variety of substrates to produce a blue oxygenindicator film, which, when activated by UV light, becomescolourless. The activated, that is, UV-photobleached, filmremains colourless unless, or until, exposed to oxygen, atwhich point the reduced methylene blue is reoxidised backto its original blue form.

The basic working principles, by which such an irre-versible oxygen indicator works, are illustrated in Figure 1.Thus, upon UVA irradiation, ultraband gap illumination(hν) of the TiO2 semiconductor particles create electron-holepairs, TiO2

∗(e−,h+). The photogenerated holes, h+, oxidisethe mild sacrificial electron donor (SED) present, glycerol inthis case, in the ink film and the remaining photogeneratedelectrons, that is, e− or TiO2

−, as in Figure 1, reduce theredox-sensitive dye, DOX to a reduced form, DRed, whichhas a different colour to DOX. In an ink film, the above

key components are encapsulated in a polymer, such ashydroxyethyl cellulose (HEC) that is soluble in a commonsolvent (usually water). Thus, UV irradiation causes an O2-sensitive ink film to change colour (step 1, Figure 1). In theabsence of oxygen, the photobleached dye will stay in thisreduced—usually colourless—state indefinitely. However,upon exposure to oxygen, it is reoxidised to its original,highly coloured form (step 2, Figure 1).

In this paper, we describe a very oxygen sensitiveversion of this type of UV-activated indicator, in which thesacrificial electron donor is glycerol and the redox dye isindigo-tetrasulfonate (ITS). The latter is highly coloured andnonluminescent in its oxidised state but virtually colourless,highly luminescent, and very oxygen-sensitive in its reducedstate, leuco indigo-tetrasulfonate (leuco-ITS).

2. EXPERIMENTAL

2.1. Materials

Unless stated otherwise, all chemicals were purchased fromAldrich Chemical Company (Gillingham, Dorset, UK). Thesemiconductor, titanium dioxide (TiO2), was P25 providedby Degussa (Frankfurt, Germany) and comprised particlesca. 30 nm in diameter, with an 80 : 20 anatase : rutile crystalphase composition.

2.2. Preparation of indigo-tetrasulfonate (ITS)based films

A typical example of an UV-activated luminescence/colourimetric oxygen-sensitive, ITS-based indicator ink,used to make the indicator films reported in this work, wasprepared by adding 200 mg of glycerol to 2 g of a 5% wthydroxyethyl cellulose (HEC) aqueous solution, to which20 mg of P25 TiO2 and 5 mg of the redox indicator indig-otetrasulfonate (ITS) had been added. The resulting mixturewas stirred magnetically for 15 minutes, followed by 15minutes sonication to disperse the usually aggregated titaniaparticles, followed by a further 15 minutes stirring. Typically,an oxygen indicator film was prepared by placing 3-4 drops(ca. 0.4 ml) of this ink onto a cut-glass microscope slide(0.8× 3.8 cm), which was subsequently spun at 2500 rpm for15 seconds. The resulting blue, transparent film was allowedto dry in the dark for 30 minutes before use.

2.3. Methods

All UV/V is spectra and absorbance versus time profileswere recorded using a Cary 50 BioVarian spectrophotometer.Luminescence spectra and luminescence intensity versustime profiles were recorded using a PerkinElmer LS50 flu-orimeter and lifetime measurements were made with an IBHFluorocube time-correlated single-photon counting system,using a NanoLed-03 source, which has its excitation peakwavelength at 370 nm. All UV irradiations were conductedusing a 2 × 4 W BLB handheld UVA light source (typicalirradiance = 4 mW cm−2).

Andrew Mills et al. 3

1

0.8

0.6

0.4

0.2

0

Abs

orba

nce

300 400 500 600 700 800

Wavelength (nm)

0200400600

Inte

nsi

ty(a

.u.)

Wavelength (nm)

300 370 440 510 580

Figure 2: UV/Vis spectra recorded for an indigotetrasulfonate(ITS) deoxygenated aqueous solution 2.8 × 10−5 mol dm−3, pH4):broken line. Upon reaction with a zinc amalgam pellet, the blueITS solution is reduced to leuco-indigotetrasulfonate (leuco-ITS),signalled by the decrease in absorbance at 590 nm and the increaseat 390 nm (spectra were recorded every 30 minutes). The insertdiagram illustrates the excitation spectrum (solid line; λem =485 nm) and emission spectrum broken line; λex = 390 nm) of a2.8 × 10−5 mol dm−3 leuco-ITS deoxygenated aqueous solution, atpH4.

3. RESULTS AND DISCUSSION

3.1. Chemical reduction of an ITS solution

An aqueous solution of leuco-ITS can be readily preparedfrom an ITS solution via the addition of a reductant, suchas zinc amalgam. However, leuco-ITS is very oxygen-sensitiveand readily oxidized back to ITS by ambient oxygen and so,in order to prevent this re-oxidation step occurring whilstrecording the spectral properties of the reduced redox dyein solution, any oxygen needs to be removed. Thus, in atypical experiment, a dilute solution of ITS (2.8 × 10−5),at pH4, was prepared and the absorbance spectra recorded,as illustrated in Figure 2 (broken line). ITS absorbs in theblue region of the visible spectrum, with a wavelength ofmaximum absorbance, λmax, at 590 nm, which is typical forthis dye [14]. This solution was then deoxygenated via 5cycles of a freeze-thaw process. Addition of a zinc-amalgampellet to the deoxygenated ITS solution reduced the highlycoloured ITS solution to its very pale yellow leuco-ITS form,as shown by the decrease in absorbance at 590 nm and theincrease in absorbance at 385 nm in Figure 2 (spectra wererecorded every 30 minutes). Excitation of leuco-ITS at thiswavelength (385 nm) revealed a blue luminescence with anemission maximum at 485 nm. The uncorrected excitation(λem = 485 nm) and emission spectrum (λexcit = 390 nm) ofthe leuco-ITS aqueous solution are illustrated in the insertdiagram of Figure 2. Single-photon counting revealed thelifetime of luminescence of leuco-ITS in aqueous solution, atpH4, to be 0.24± 0.02 microseconds.

3.2. Photobleaching of an ITS oxygen indicator film

A parallel study, to that above, was carried out on a typicalITS oxygen indicator film, with one set of experiments

carried out under vacuum (10−3 mbar), that is, O2 free,and the other under ambient conditions, that is, 21% O2.A typical absorbance spectrum of a blue-coloured indicatorfilm under vacuum before (broken line) and after activationwith UVA light is illustrated in Figure 3(a). From these resultsit can be seen that the initially blue-coloured ITS film isactivated, that is, converted from ITS to leuco-ITS via thephotoreduction of ITS by the UV-excited titania particles(see Figure 1) in under 3 minutes of UVA irradiation. Thisphotoreduction process (step 1, Figure 1) produces a fallin absorbance of the film at 600 nm as a function of UVAirradiation time as indicated by the data in Figure 3(a) insertdiagram. In contrast, under otherwise the same conditions,a typical oxygen indicator in an ambient environment takestwice as long to activate using the same UVA light source(I = 4 mW cm−2). The lower rate of photobleaching in thelatter system is due to the presence of oxygen in the ambientenvironment which is able to reoxidise the reduced form ofthe dye via a dark reaction, that is step 2 in Figure 1.

Figure 3(b) contains photographs of a typical TiO2/ITS/glycerol/HEC ink spun coated onto a coverslip (i) beforeand (ii) after photobleaching by UV irradiation (30 seconds,I = 7 mW cm−2) in air. Photograph (iii), in Figure 3(b), isafter ca. 15 minutes in the dark under ambient conditions,during which time the ITS film regains its original colour(and loses its luminescence), due to the reoxidation of leuco-ITS to ITS by oxygen. Photographs (i)–(v) confirm that theITS indicator films can be photoactivated under ambientconditions to produce a luminescent yellow, leuco-ITS filmthat is oxygen-sensitive.

The UV activation of a typical ITS oxygen indicatorfilm can also be monitored by luminescence, given thatleuco-ITS luminesces at 470 nm in the film (see Figure 3(b),photographs (iv) and (v)). The observed change in theemission spectrum of an ITS oxygen indicator film, undervacuum, upon exposure to UVA light as a function of time isillustrated in Figure 4. The insert diagram illustrates a plot ofthe variation in a film’s luminescence intensity as a functionof UVA irradiation time, and, as in the absorption spectralchanges in Figure 2, shows that in the absence of oxygen thefilm is activated in less than 3 minutes, in comparison toan oxygen indicator film in an ambient environment, whichtakes ca. 6 minutes due to the dark back reaction, that is, step2 in Figure 1.

3.3. Dark oxidation of an activated ITS film

Once activated, that is, photobleached, via step 1 in thereaction scheme in Figure 1, the stability of the photo-bleached oxygen indicator film depends on the level ofoxygen present in the environment in which it finds itself.Thus, in the dark and in the absence of oxygen, the UV-activated oxygen indicator remains bleached indefinitely,whereas in the presence of oxygen, its colour is restoredwithin seconds and the luminescence associated with thereduced form of ITS is quenched. This is nicely illustrated bythe data in Figures 5 and 6 (broken lines), which depict therecorded change in absorbance at 600 nm and luminescenceintensity at 470 nm, respectively, as a function of time, of an

4 International Journal of Photoenergy

0.5

0.4

0.3

0.2

0.1

Abs

orba

nce

300 400 500 600 700 800

Wavelength (nm)

0

0.4

0.8

Rel

.(Δ

abs.

)

Irradiation time (s)0 40 80 120 160

(a)

(i) (ii) (iii)

(iv) (v)

(b)

Figure 3: (a) Recorded change in the UV/Vis spectrum of an ITS oxygen indicator film (broken line), in the absence of oxygen, uponexposure to UVA light (intensity: 4 mW cm−2) as a function of time. The spectra illustrated were irradiated (from top to bottom at 600 nm)for 0 second up to 160 seconds, in 10-second intervals. The insert diagram illustrates the change in absorbance of the indicator film at 600 nmas a function of irradiation time, derived from the data in the main diagram. (b) Photographs of an oxygen indicator film on a coverslip. (i)TO2/ITS/glycerol/HEC film before and (ii) after 30 seconds UVA irradiation (I = 7 mW cm−2), and (iii) film in photograph (ii) allowed torecover after 15 minutes in air. (iv) Photograph of film (i) under very low UV-light illumination in the presence of oxygen. (v) Photographof film (ii) also under very low UV-light illumination.

ITS-based oxygen indicator film that has been UV irradiatedin a vacuum and subsequently exposed to an ambientatmosphere by opening up the system to air. In contrast tothis data, the solid lines in Figures 5 and 6 correspond to therecovery of an UV-activated oxygen indicator film, irradiatedunder ambient conditions (i.e., 21% O2) and maintainedin this environment once the photobleaching process wasstopped. Not surprisingly, the oxygen indicator film in anevacuated environment recovered its original blue colourwhen exposed subsequently to oxygen in the dark. However,the ITS indicator film which was initially photobleached inair recovered only ca. 50% of its original colour, possiblydue to some photodegradation of the redox dye, presumablycaused by the longer exposure to UVA light in air neededto photobleach the film, which probably promotes dyedegradation via singlet oxygen production.

The insert diagrams in Figures 5 and 6 show that thedark process (step 2, Figure 1) gives a good fit to first-order kinetics for a UV-activated oxygen indicator film whenphotobleached and left to recover in an ambient atmosphere,or when photobleached in a vacuum and subsequentlyexposed to air. Such first-order kinetics are typical for UV-activated oxygen film indicators and provide support forthe proposed scheme, that the kinetics of step 2 in thereaction scheme in Figure 1 depend directly upon the rateof diffusion of O2 through the film. Other work indicatesthat the rate of this dark recovery process is first order withrespect to the partial pressure of oxygen in the ambientatmosphere as with similar UV-activated, oxygen-sensitivefilm indicators. The first-order rate constants for the recoveryof a typical UV-activated oxygen indicator film in an ambientenvironment and in a vacuum are given in Table 1. It remains

300

250

200

150

100

50

0

Inte

nsi

ty(a

.u.)

400 450 500 550 600

Wavelength (nm)

0

0.4

0.8

Rel

.(ΔI)

Irradiation time (s)0 50 100 150

Figure 4: Recorded change in the emission spectrum of an ITSoxygen indicator film (broken line), under vacuum, upon exposureto UVA light as a function of time. The spectra illustrated wereirradiated (from bottom to top) for 0 second up to 140 seconds,in 10-second intervals. The insert diagram illustrates the change inluminescence intensity of the indicator film at 470 nm as a functionof irradiation time, derived from the data in the main diagram.

unclear why the kinetics of step 2, the dye recovery step, areslower for an ITS film irradiated in air, compared to thatfor films irradiated in a vacuum, although an appreciabledegree of dye degradation is observed for the latter process(vide supra). The above results stress the need for oxygen-freeconditions when UV-activating these ITS-based indicators.

4. CONCLUSION

A novel, fast acting luminescent and colourimetric oxygenindicator ink is described, containing a sacrificial electron

Andrew Mills et al. 5

Table 1: Photobleaching and recovery kinetics of a typical UV-activated oxygen indicator.

Photobleaching kinetics Recovery kinetics

Environment Abs·k1(s−1) r2 Int·k1(s−1) r2 Abs·k1(s−1) r2 Int·k1(s−1) r2

Ambient 0.0063 0.99 0.0075 0.96 0.0062 0.99 0.0091 0.99

Vacuum 0.013 0.99 0.015 0.97 0.033 0.99 0.027 0.98

0.42

0.39

0.36

0.33

0.3

Abs

600

0 100 200 300 400 500 600

Time (s)

−1−0.8−0.6−0.4−0.2

0ln

(rel

.Δab

s 600

)

Time (s)0 20 40 60 80 100 120

Vacuum

Air

Figure 5: Change in the absorbance, at 600 nm, of a typical UV-activated ITS oxygen indicator film as a function of time, underan ambient atmosphere (solid line) and an ITS film UV-activatedunder vacuum and subsequently exposed to air (broken line). Eachfilm was fully photobleached, using UVA light, before being allowedto recover. The insert diagram is a first-order plot (natural log of thechange in the absorbance, that is, ln (ΔAbs), at 600 nm versus time),over one half life, derived from the data in the main diagram. Therate constants for this recovery step are given in Table 1.

350

300

250

200

150

100

50

0

Inte

nsi

tyλ m

ax

0 100 200 300 400 500 600

Time (s)

−1−0.8−0.6−0.4−0.2

0

lnre

l.(Δ

int λ

max

)

Time (s)

0 20 40 60 80 100

VacuumAir

Figure 6: Change in the luminescence intensity, at 470 nm, of atypical, UV-activated ITS oxygen indicator film as a function oftime, under an ambient atmosphere (solid line) and when exposedto air (broken line), having been first UV-activated under vacuum.Each film was fully photobleached, using UVA irradiation, beforebeing allowed to recover in air. The insert diagram is a first-orderplot, over one half life, derived from the data in the main diagram.

donor (glycerol) and the redox indicator (indigotetrasul-fonate). The indicator is blue coloured in the absence ofUV light, however, upon exposure to UV light it not onlyloses its colour but also luminesces, unless and until itis exposed to oxygen, whereupon, under dark conditions,

its colour is restored. The initial photobleaching spectral(absorbance and luminescence) response characteristics inair and in vacuum are described and discussed in termsof a simple reaction scheme involving UV activation of thetitania photocatalyst particles, which are used to reduce theredox dye, ITS, to its leuco form. The response characteristicsof the activated, that is, UV-photobleached form of theindicator, to oxygen are also reported. This indicator appearsmore sensitive towards oxygen than previous UV-activatedindicators, based on methylene blue, probably due to thelower redox potential [14] of ITS (−0.046 V versus SHE)compared to that of methylene blue (+0.028 V versus SHE)at pH7. This indicator appears susceptible to apprecia-ble photodegradation when UV-activated under ambient(21% O2) conditions. In the absence of oxygen, however,UV activation is not only more rapid, but also does notproduce any significant photodegradation. Thus, the inkappears particularly suited for use in systems that are usuallyoxygen free, before UV activation, as a means of indicatingany subsequent leak or tampering.

ACKNOWLEDGMENTS

The authors are pleased to acknowledge a grant to C.Tommons from Bell College Research Grants Committeeand M. C. Tedford thanks the Royal Society of ChemistryResearch Fund.

REFERENCES

[1] M. L. Rooney, Active Food Packaging, Blackie, London, UK,1995.

[2] A. L. Brody, B. R. Strupinsky, and L. R. Kline, Active Packagingfor Food Applications, Technomic Publishing, Lancaster, Pa,USA, 2001.

[3] M. L. Hitchman, Measurement of Dissolved Oxygen, JohnWiley & Sons, New York, NY, USA, 1978.

[4] S. J. Valenty, “Gas chromatographic determination of dis-solved hydrogen and oxygen in photolysis of water,” AnalyticalChemistry, vol. 50, no. 4, pp. 669–671, 1978.

[5] O. S. Wolbeis, Fibre Optic Chemical Sensors, CRC Press, BocaRaton, Fla, USA, 1991.

[6] OxySense, Inc., March 2007, http://www.oxysense.com/.[7] M. Goto, “Oxygen Indicator,” 1987, JP Patent 62259059.[8] Y. Yoshikawa, T. Nawata, M. Otto, and Y. Fujii, “Oxygen

indicator,” 1979, US Patent 4169811.[9] E. S. Davis and C. D. Garner, “Oxygen indicating composi-

tion,” 1996, UK Patent 2298273.[10] K. C. Krumhar and M. Karel, “Visual indicator system,” 1992,

US Patent 5096813.[11] Mitsubushi Gas Chemical Company, Inc., March 2007, http://

www.mgc.co.jp/eng/company/materials/products/ageless/related/index.html.

6 International Journal of Photoenergy

[12] S.-K. Lee, A. Mills, and A. Lepre, “An intelligence ink foroxygen,” Chemical Communications, no. 17, pp. 1912–1913,2004.

[13] S.-K. Lee, M. Sheridan, and A. Mills, “Novel UV-activatedcolorimetric oxygen indicator,” Chemistry of Materials, vol. 17,no. 10, pp. 2744–2751, 2005.

[14] E. Bishop, Indicators, Pergamon Press, Oxford, UK, 1972.

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